1. Field of the Invention
This invention relates to an electrically discharging method and device utilizing AC corona discharge.
2. Description of the Prior Art
As a device for removing imparted charge from the surface of a positively or negatively charged insulative material or electrophotographically sensitive plate (hereinafter referred to as the charged body, which means the body to be discharged), there has hitherto been a device in which an AC corona discharger is disposed in opposed relationship with the charged body and an AC high voltage is supplied to the discharger as it is moved relative to the charged body.
Also, in some of the well-known image transfer type electrophotographic apparatuses wherein a photosensitive medium is repetitively used to produce copies, the surface of the photosensitive medium is discharged after completion of a sequence of processes of charging, exposure, development and image transfer and before stoppage of the apparatus, thereby preventing irregularities of image from being created during the subsequent sequence of processes. See U.S. Pat. No. 3,698,926, for example.
However, AC corona discharge, if stopped immediately after the discharging operation, might sometimes cause some portions of the charged body to be non-uniform in potential. This will be explained by reference to FIG. 1 of the accompanying drawings which shows an electric circuit equivalent to a AC corona discharge.
In FIG. 1, R1 corresponds to the resistance of the space between a corona discharger and a charged body, and C1 corresponds to the electrostatic capacity between the charged body and the ground. The state of the charged body having been charged corresponds to the state of the capacitor C1 having been charged. If corona discharge is then effected actively, the resistance R1 of the space will be reduced to permit the release of the charge so far stored in the electrostatic capacity C1 of the charged body.
On the other hand, in a state that AC corona discharge is still continued after the discharging has been completed, the potential difference Vc across the capacitor C1 is expressed as V.sub.c =A/jWCR1+1 sin Wt (j in the unit of imaginary number), where the applied voltage V is V=A sin Wt (A is the maximum value). In this equivalent circuit, the amplitude of the potential difference across the capacitor is 2A/.sqroot.1+W.sup.2 C.sup.2 R1.sup.2. Thus, after completion of the discharging, the surface potential of the charged body may be regarded as having been varied in the range from an upper to a lower limit determined by .+-.A/.sqroot.1+W.sup.2 C.sup.2 R1.sup.2.
Therefore, if corona discharge is abruptly discontinued with the charged body being opposed to the discharger, the then surface potential is somewhere in the range from the upper to the lower limit determined by .+-.A/.sqroot.1+W.sup.2 C.sup.2 R1.sup.2 and this will later present itself as a portion of non-uniform potential.
This will further be considered. Particularly, in the case of an apparatus wherein an electrostatic latent image is formed on an insulative plate while during rotation, whereafter the latent image is developed into a visible image, which is then transferred to transfer paper and the insulative plate is subjected to AC corona discharge for removal of the charge therefrom and thus for reuse of the insulative plate, there is left in the insulative layer a peak potential of the aforementioned upper or lower limit when the apparatus is stopped from operating. This will result in irregularities of image when a subsequent processing cycle is executed.
Also, in an apparatus wherein a photosensitive plate comprising, as viewed from the surface thereof, an insulative layer, a photoconductive layer and a conductive layer, is subjected to primary charge and simultaneous exposure and AC discharging while being rotated to thereby form an electrostatic latent image on such photosensitive plate, whereafter the contrast of the latent image is increased by total surface exposure and then developed during the next step, followed by subsequent steps similar to those mentioned above for obtainment of a transfer image, there is created in the insulative layer a peak potential corresponding to the peak value of the AC field which leads to irregularities of the internal field within the photoconductive layer which is CdS or the like. Thus, the copying apparatus is left with an internal memory created therein. Such memory could result in irregularities of image unless it is removed before the next cycle of process is executed. The memory could not completely be erased without operation of the primary charger and AC discharger and without several tens of full rotations of the photosensitive medium.